The event which brings together people from across the garden industry will "explores the challenges and opportunities that face garden retail now and in the future" the HTA said.
HTA chief executive Carol Paris said: "We have decided to move HTA Garden Futures out of London for the first time in a number of years and Heythrop Park provides a stunning venue in which to do this.
"By running the conference and dinner on the same day and in the same location delegates can really maximise the opportunity that the event provides."
The HTA's catering conference, re-arranged from February to this summer, will now last two days at a place and on a date to be announced next week.
GIMA's AGM and conference is on 15 April at a venue to be confirmed.